Programmable thermostats have gained wide popularity in the last decade because of microprocessor cost reduction, the ease of programming these small microprocessors, and the cost savings and comfort enhancement of programmed temperature adjustments with parameters such as time of day, outside temperature, season of year, and other parameters. Such low cost microprocessors have the capability of functions in excess of those currently being utilized and provide an opportunity for enhanced heating-cooling and fan control not currently utilized at a small incremental cost.
The present invention deals with the circulation of air within the building and particularly the mixing of hot and cold air within the space and the environmental cleanliness of the air. In moderate outside temperatures, in the range of 35 to 80 degrees, the off cycles in both the heating and cooling modes will increase over the bordering colder or hotter ranges. This long off cycle time stagnates the room air and reduces air cleanliness because filtration is off during these periods between heating and cooling cycles. Typically the fan is on during the heating cycle and off as the cycle ends, and the fan is on during the cooling cycle and remains on at the termination of the cooling cycle to recapture residual cooling. So it is known in the industry to extend the fan cycle to increase thermal efficiency. But that is only part of the objective of the present invention. Extending fan cycles past the low or high temperature shut-off does not ameliorate the stagnant air after long periods of cycle shut-off nor does it improve air quality during these off periods.
There have been several systems that provide independent or dependent fan cycles; for example, the Noto, U.S. Pat. No. 5,582,233; the Toth, U.S. Pat. No. 6,318,639; and the Meyer, et al., U.S. Pat. No. 5,765,636.
In the Noto '233 patent, the fan control circuitry 20 is completely independent from the thermostat 46, and thus adds a considerable cost to the system including duplicating a power supply, such as indicated at 26, and requires new connections at FR and Fan-G into the system. Furthermore, the fan in Noto operates its duty cycle at 1½ minutes every 15 minutes, which is too short a time period and too frequent to increase the system efficiency.
In the Toth '639 patent, assigned to Emerson Electric, the fan duty cycle circuitry is contained in the thermostat, but it has an extremely complicated variable “on” time for the fan which requires excessive and unnecessary software. Further, in Toth, the fan mode overrides the normal heat-cool cycle control subjecting the room space to abnormally hot or cool conditions.
In the Meyer, et al. '636 patent, the fan control circuitry 20 is again separate from the thermostat 10. The Meyer, et al. patent is mainly concerned with preventing short cycling of the fan adjacent the heat-cool cycles. There is a brief mention of independent fan operation in Meyer, et al., but this description is silent as to frequency of cycling and duration of cycling so that Meyer, et al. is not deemed pertinent to the present invention.
It is a primary object of the present invention to ameliorate these problems noted above and provide a programmable thermostat with a fan recirculating mode that is independent of the heat-cool and off modes of today's air conditioning systems.